Electrical measuring apparatus



Feb. 7, 1939. .w. s. JENNENS ET AL 2,146,073

ELECTRICAL MEAS UR ING APPARATUS Filed Jan 22, 1937 ATTORN/EY Patented Feb. 7, 1939 UNITED STATES PATENT oFFicr.

ELECTRICAL MEASURING APPARATUS New Jersey Application January 22, 1937, Serial No. 121,901

1 Claim.

This invention relates to electrical measuring apparatus and particularly to apparatus for measuring electrical quantities which affect the natural period of oscillation of an electrical circuit such as inductance and-capacitance.

quire a high degree of technical skill for operation, and in which the readings may be made visually.

A further object of the invention is to provide electrical measuring apparatus in which the chservations may be made by means of a tuning indication tube.

. 'A further object of the invention is to provide a measuring circuit in which the frequency of oscillation of one circuit is influenced by the test piece, and in which said frequency of oscillation is compared to a reference circuit of known frequency.

A further object of the invention is to provide testing apparatus which is economical to manufacture, which is easily portable and by which accurate measurement may be made without a high degree of technical skill,

A further object of the invention is to provide measuring apparatus which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

5 The invention is exemplified by the combination and arrangement of' parts shown in the a part of the oscillating circuit, and means for 'visually observing when the test oscillator is 50 tuned to exactly the same frequency as the reference oscillator.

The reference oscillator is shown in the upper portion of Fig. 1 of the diagram and comprises an oscillatory circuit including inductance l0 and 55 capacitance II. An adjustable capacitance I2 is Fig. 1 is a circuit diagram showing one emconnected in parallel with the capacitance H to provide for slight adjustment of the reference frequency. Oscillation of the. reference circuit is maintained by means of a vacuum tube l3 having a filament M, a grid 15, and a plate Hi. The filament i4 is connected through its terminals I! to any source of heating current such as the secondary I 8 ofv a transformer, the primary 19 of which is; energized from an ordinary 1l0-volt A. C. lighting circuit. The grid i5 is connected by means of a grid leak and condenser 20 with one terminal of the inductance ill and one terminal of the filament I4 is connected by a lead 2| to an intermediate point on the inductance Ill.

The plate I6 is connected through a capacitance 22 to the end of inductance Ill opposite that connected to the grid. The plate i6 is also connected through a choke coil 23 to a direct current source of voltage which may be supplied by a well known form of rectifier circuit 25 with the requisite secondary windings on the core 26 of the transformer of which 19 is the primary.

The reference oscillator may be any one of many different kinds that have definite periods of oscillation. The one illustrated in the diagram employs the well known standard Hartley circuit. The arrangement and operation of this circuit is more fully described in any standard radio handbook. The frequency of oscillation of the circuit is determined by the inductance the frequency of oscillation. The one shown in the diagram comprises a standard Hartley circuit in which the frequency of oscillation is determined by inductance 21, capacitance units 28 and 23, and other capacitance units connected to the circuit through a transformer 30, the operation of which will be more fully described.

Oscillation of the test oscillator is maintained by a vacuum tube 31 similar to the tube l3 and 'connected through lead 32 to the rectifier 25.

Capacitors 33, 34, 35, 36, and 31 may be connected in certain combinations by means of a switch 38 to the secondary of the transformer 30.v The capacitors 33 and 34 are high grade standard devices and are adjusted to a fixed known value by means of the variable unit 33 and when once so adjusted are sealed against further adjustment, so that they constitute a fixed capacitance of known value. 31 is the test piece, the capacitance of which is to be measured. 35 and 35 are adjustable capacitors, controlled by'a single operating wheel provided with graduated scales, which are calibrated to indicate the capacitance 6f the test piece.

A pickup coil or secondary 40 is associate with the inductance I0 and is connected in circonnected together at 44. Means are provided for observing when the reference circuit and test circuit are oscillating at the same frequency by comparing the potential of intermediate points on the two potentiometers 4|. and 43. One form of indicator well suited for this purpose is a tuning indicator tube 45 commonly designated in the trade by the designation No. 6E5, and

sometimes referred to as an electric eye. The tube is commonly used in radio receiving circuits for indication of optimum tuning of receiving sets to the various frequencies of different broadcasting stations. The tube consists essentially of a triode unit and an electron ray device.

It is so designed that a zero voltage between the grid and cathode of the triode unit produces a dark sector on the target of the cathode ray portion of the tube, and as a negative potential is'applied, the black sector closes up to a narrow line, If alternating current is applied, the sector will close on the negative half cycle and remain open on the positive. The grid and cathode of the tube are connected respectively to the potentiometers -4| and .42 at the proper positions to give equal voltage. When the frequencies of oscillation of the two oscillators are identical and they are 180 degrees out of phase, the voltage between the grid and cathode of-the tube is zero nd the black sector on the target remains wide pen. This gives a very sensitive indication since the slightest difference in frequency will not 'give a stationary wide open sector. Ifthe frequencies are slightly diife'rent, the black sector onthetarget will open and close at a frequency which is the difference between two oscillation frequencies. It is very easy, therefore, for even an unskilled observer to determine when the two oscillators are tuned to the same frequency.

In operation, the apparatus is connected up as shown in Fig. 1 with the switch 33 in the position indicated. In this position the capacitor 33-34 is connected to the secondary of the transformer 30 and the capacitors 35, 33, and 3'! are disconnected. The variable capacitor I! may now be adjusted if required to give the reference oscillator the same frequency as the test oscillator. The switch may now be shifted to the right, which will disconnect the standard capacitor 33-34 and substitute therefor the capacitor 35 in parallel with the test piece 31. The capacitor 35 is then adjusted until the test circuit is again brought to the same frequency as the reference circuit, which of course,'isthe same frequency it had with the standard capaci-, tor 33-34 in the circuit. When this adjustment for the. standard capacitor 50.

value of the capacitance of the test piece directly, or the capacitance of the test piece may be obtained by referring the scale readings to a calibration curve.

In order to extend the range of the instrument, 9. second adjustable capacitor 35 is pro- I vided, which may be connected in parallel with the capacitor 35 by shifting the switch 33 to the left instead of the right. The two capacitors 35 and 35 are preferably operated by the same handle or hand wheel which may be provided with two scales, one for use when the switch handle is moved to the right, and the other for use when the switch handle is moved to the left. The effect of the capacitors in the circuit controlled by the switch 35 is multiplied by the transformer 30, the effect being proportional to the square of the transformer ratio. It is usually desirable toground one terminal of the test piece as indicated at 45. The transformer 30 makes it possible to do this without placing a ground on the test oscillator circuit, which would short-circuit a portion of the inductance 21,

In the diagram shown in' Fig.2 the circuit is represented in the form of a bridge having thereference oscillator connected in one leg of the bridge through a pick up coil 5| and the test oscillator 52 is connected in another leg of the bridge through a-pick up ,coil 53. Impedances 54 and 55 correspond to the parts of the impedance 4| in Fig. 1 of the drawing,and im- 'of this conductor are connected to a common point. The indicator 59 connects intermediate points of the potentiometers formed by the impedances 54 and 55 and the impedances 55 and 51. 'In place of a rectifier, the energy for the various tubes and for the oscillators may be supplied from batteries as shown in this figure.

In operation, a standard capacitor 50 is connected in the circuit by the switch 5| and a capacitor 52 is adjusted to bring the reference oscillator into synchronism with the test oscillator. The switch 5| is then moved to substitute the test piece 53 and the adjustable capacitor 54 The capacitor 54 is then adjusted to restore the frequency of the oscillator 52 to its original value as com-' pared with'the reference oscillator 50. The capacitance of the test piece 53 may then be de-' termined from the scale reading of the variable capacitor 54 in the manner discussed in connectionwith Fig. 1.

Various modifications of the invention will readily suggest themselves to those skilled in the art, and we donot wish therefore to be restricted to the particular forms shown and described, except as set forth in the following claim.

We claim:

Measuring apparatus comprising a test oscillator having a circuit energized thereby, a standard reactance for. connection in said circuit to impart a predetermined frequency of oscillation thereto, means for connecting an unknown reactance and an adjustable reactance in parallel with each other in said circuit in lieu of said standard reactance, a reference oscillator circuit having an adjustable reactance therein, and

means for comparing the frequency of said test oscillator circuit and said reference oscillator cir-' cuit to facilitate adjusting said reference circuit tron ray tuning indicator tube having its grid and cathode connected respectively each to a point in one of said circuits, and impedance in series in said circuits respectively between said common point and the respective points of con- 6 nection with said grid and cathode.

WALTER S. JENNENS. CHARLES J. LHLLER, JR. 

